The present invention relates to a control system of torque distribution to the left and right driving wheels of a motor vehicle, and more particularly to a control system in which output torque of a transmission is distributed to the left and right rear driving wheels in accordance with driving conditions of the vehicle.
A differential of the motor vehicle is provided with a differential restricting device such as a fluid operated multiple-disk clutch which operates to lock the differential when one of the left and right wheels slips. Thus, it is possible to drive the vehicle without the vehicle becoming stuck. On the other hand, when the differential is locked, steerability of the vehicle decreases. In order to solve such a problem it has been proposed to change the clutch torque for restricting the differential operation in accordance with slipping states, for example the difference between the speeds of the left and right driving wheels.
However, the change in the clutch torque causes a change in the distribution ratio of the torques to the left and right wheels, thereby affecting the running behavior of the vehicle. Namely, if the clutch torque is zero, the fluid operated clutch is free, so that in the case of a bevel gear differential, a torque TRL transmitted to the left wheel and a torque TRR transmitted to the right wheel are equal (TRL:TRR=50:50) irrespective of a friction coefficient .mu. against the road surface. Hence, stable driving is enhanced while maintaining the steerability. On the other hand, as the clutch torque for restricting the differential operation increases, the torques are distributed at a ratio in accordance with a left dynamic weight WTL, a right dynamic weight WTR, a friction coefficient .mu.L at the left wheel, and a friction coefficient .mu.R at the right wheel as follows. EQU TRL:TRR=WTL.multidot..mu.L:WTR.multidot..mu.R
Accordingly, if the dynamic weight WTL and WTR are equal to each other, a larger torque is distributed to the wheel which runs on the surface having a larger friction coefficient .mu.. As a result, the vehicle body swings laterally, causing unstable driving as well as a decrease in steerability. Thus, it is necessary to control the torque distribution ratio when driving on a road having a small friction coefficient .mu. in dependency on the steering angle and vehicle speed. Moreover, it is necessary to determine the differential operation restricting clutch torque in consideration of the steerability and the driving stability.
Japanese Patent Application Laid-Open 61-132420 discloses a system for controlling the torque distribution where the differential operation restricting clutch torque is increased with increases in the vehicle speed and the steering angle. However, although the slipping of the wheels in a high vehicle speed range can be prevented by locking the differential, the torque distribution deviates to one of the left or right wheels in accordance with the left friction coefficient .mu.L and the right friction coefficient .mu.R. Thus, the driving stability of the vehicle when driven straight-ahead and the steerability at cornering at high speed deteriorate. In addition, at a large steering angle, the differential restriction is slightly affected, thereby decreasing the steerability.
It has been considered to provide a viscous coupling as a device for restricting the differential operation. However, since the viscous coupling has differential restricting characteristics which change with the change of the speed difference between the left and right wheels in a convex curve, a proper restriction effect can not be obtained.